Extract JSON_CPU_LITTLE_ENDIAN_64BITS definition

Only apply these definitions on 64 bits archs, as it's unclear
if they have performance benefits or compatibility issues on
32bit archs.

Author: byroot

ext/json/ext/json.h

@@ -3,6 +3,7 @@
 
 #include "ruby.h"
 #include "ruby/encoding.h"
+#include <stdint.h>
 
 #if defined(RUBY_DEBUG) && RUBY_DEBUG
 # define JSON_ASSERT RUBY_ASSERT
@@ -82,4 +83,10 @@ typedef unsigned char _Bool;
 #endif
 #endif
 
+#if defined(__BYTE_ORDER__) && __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ && INTPTR_MAX == INT64_MAX
+#define JSON_CPU_LITTLE_ENDIAN_64BITS 1
+#else
+#define JSON_CPU_LITTLE_ENDIAN_64BITS 0
+#endif
+
 #endif // _JSON_H_

ext/json/ext/parser/parser.c

@@ -323,7 +323,7 @@ typedef struct JSON_ParserStateStruct {
     int current_nesting;
 } JSON_ParserState;
 
-static inline ssize_t rest(JSON_ParserState *state) {
+static inline size_t rest(JSON_ParserState *state) {
     return state->end - state->cursor;
 }
 
@@ -525,7 +525,7 @@ json_eat_whitespace(JSON_ParserState *state)
                 state->cursor++;
 
                 // Heuristic: if we see a newline, there is likely consecutive spaces after it.
-#if defined(__BYTE_ORDER__) && __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+#if JSON_CPU_LITTLE_ENDIAN_64BITS
                 while (rest(state) > 8) {
                     uint64_t chunk;
                     memcpy(&chunk, state->cursor, sizeof(uint64_t));
@@ -966,7 +966,7 @@ static inline VALUE json_parse_string(JSON_ParserState *state, JSON_ParserConfig
     return Qfalse;
 }
 
-#if defined(__BYTE_ORDER__) && __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+#if JSON_CPU_LITTLE_ENDIAN_64BITS
 // From: https://lemire.me/blog/2022/01/21/swar-explained-parsing-eight-digits/
 // Additional References:
 // https://johnnylee-sde.github.io/Fast-numeric-string-to-int/
@@ -995,8 +995,8 @@ static inline int json_parse_digits(JSON_ParserState *state, uint64_t *accumulat
 {
     const char *start = state->cursor;
 
-#if defined(__BYTE_ORDER__) && __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
-    while (rest(state) >= 8) {
+#if JSON_CPU_LITTLE_ENDIAN_64BITS
+    while (rest(state) >= sizeof(uint64_t)) {
         uint64_t next_8bytes;
         memcpy(&next_8bytes, state->cursor, sizeof(uint64_t));